Communication devices such as User Equipments (UE) are enabled to communicate wirelessly in a radio communications system, sometimes also referred to as a radio communications network, a mobile communication system, a wireless communications network, a wireless communication system, a cellular radio system or a cellular system. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
User equipments are also known as e.g. mobile terminals, wireless terminals and/or mobile stations, mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity.
The wireless communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a network node such as a Base Station (BS), e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. eNB, eNodeB, NodeB, B node, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several radio access and communication technologies. The base stations communicate over the radio interface operating on radio frequencies with the user equipments within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: GroupeSpécial Mobile).
In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the user equipment. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
In order to achieve very high data rates in LTE or LTE Advanced communications systems, it is necessary to increase the transmission bandwidths over those bandwidths that may be supported by a single carrier or channel. The method is termed carrier aggregation.
In carrier aggregation, the channels or carriers may be in contiguous elements of the spectrum, or they may be in different frequency bands.
Carrier aggregation is supported by both Frequency-Division Duplexing (FDD) LTE and Time-Division Duplexing (TDD) LTE. This ensures that both FDD LTE and TDD LTE are able to meet the high data throughput requirements that are placed upon them.
When carriers are aggregated, each carrier is referred to as a Component Carrier (CC). The component carrier may be divided into two different categories; primary component carrier and secondary component carrier. The primary component carrier is the main carrier in any group. There will be a downlink primary component carrier and an associated uplink primary component carrier. Further, there may be one or more secondary component carriers.
Accordingly, two categories of cells are defined for carrier aggregation, Primary Cell (PCell) and Secondary Cell (SCell).
The primary cell operates on a primary frequency. On the primary frequency, the user equipment either performs the initial connection (re)establishment procedure, or connect to the cell indicated as the primary cell in the handover procedure.
The secondary cell operates on a secondary frequency. The secondary frequency may be configured once a Radio Resource Control (RRC) connection is established between the user equipment and the communications system. Further, the secondary frequency may be used by the user equipment to provide additional radio resources.
For the primary cell the user equipment applies the system information acquisition and change monitoring procedures for the primary cell only. The primary cell may be regarded as the anchor carrier for the user equipment and is thus used for basic functionalities such as monitoring of radio link failures. As previously mentioned, if more than one component carrier is configured for the user equipment, the additional component carriers are denoted as secondary cells for the user equipment.
For a user equipment supporting carrier aggregation, E-UTRAN provides all system information relevant for operation in the concerned cell in the RRC_CONNECTED mode via dedicated signaling when adding a secondary cell. For secondary cells, change of system information is handled by release and addition of the concerned secondary cell. This may be done with a single RRCConnectionReconfiguration message.
In carrier aggregation, the user equipment may need to re-select primary cell due to changes in channel quality. For example, the user equipment may choose to re-select the current secondary cell as the primary cell when measurement results for the primary cell are worse than, e.g. below, a threshold while the secondary cell has a good channel quality, i.e. when the measurement result for the secondary cell is better than, e.g. above, a threshold.
For user equipments supporting carrier aggregation, E-UTRAN may change primary cell by means of a handover procedure using an RRCConnectionReconfiguration message including the mobilityControlInfo.
FIG. 1 schematically illustrates the steps 3-11 of the prior art primary cell re-selection using the handover procedure according to 36331-a50, 3GPP, Technical specification group radio access network, Evolved Universal Terrestrial Radio Access (E-UTRA), Radio Resource Control (RRC), protocol specification (Release 10). For clarity reasons, the steps 1-2 and 12-18 have been excluded in FIG. 1.
In this procedure, in the context of the user equipment choosing to re-select the secondary cell as the primary cell, a user equipment measurement report, e.g an RSRP for a serving cell and a neighbor cell, is sent from the user equipment to the serving eNB (source eNB, S-eNB), which is the eNB controlling the current primary cell. If one or more criterias for cell re-selection is met, the S-eNB sends a Handover (HO) request to the target eNB (T-eNB) in step 4 and receives an HO Request acknowledgement (ACK) from the target eNB (T-eNB) in step 6. In the described scenario, the target eNB is the eNB controlling the current secondary cell. In step 7, the S-eNB sends an RRCConnectionReconfiguration message to the user equipment, and in steps 9 and 10 the user equipment synchronizes to the T-eNB by using the Random Access Channel (RACH). After that, in step 11, the user equipment sends an RRCConnectionReconfiguration Complete message to the T-eNB and starts to use the previous secondary cell as the primary cell. This concludes the primary cell reselection procedure. After completion of the primary cell re-selection procedure, the previous primary cell may be used by the user equipment as a secondary cell. This may be achieved by including the previous primary cell into a secondary cell list Information Element (IE) comprised in the RRCConnectionReconfiguration message.
However, this process has shown to be time-consuming and the user equipment may lose its connection during this process.